This invention relates to direct electrostatic printing (DEP) devices and more particularly to wrong sign toner extraction from a toner cloud.
Of the various electrostatic printing techniques, the most familiar and widely utilized is that of xerography wherein latent electrostatic images formed on a charge retentive surface are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper.
A lesser known and utilized form of electrostatic printing is one that has come to be known as direct electrostatic printing (DEP). This form of printing differs from the aforementioned xerographic form, in that, the toner or developing material is deposited directly onto a plain (i.e. not specially treated) substrate in image configuration. This type of printing device is disclosed in U.S. Pat. No. 3,689,935 issued Sept. 5, 1972 to Gerald L. Pressman et al.
Pressman et al disclose an electrostatic line printer incorporating a multilayered particle modulator or printhead comprising a layer of insulating material, a continuous layer of conducting material on one side of the insulating layer and a segmented layer of conducting material on the other side of the insulating layer. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of the conductive material is formed around a portion of an aperture and is insulatively isolated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layer while a fixed potential is applied to the continuous conductive layer. An overall applied field projects charged particles through the row of apertures of the particle modulator and the density of the particle stream is modulated according to the the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charged particles impinge upon a print-receiving medium interposed in the modulated particle stream and translated relative to the particle modulator to provide line-by-line scan printing. In the Pressman et al device the supply of the toner to the control member is not uniformly effected and irregularities are liable to occur in the image on the image receiving member. High-speed recording is difficult and moreover, the openings in the printhead are liable to be clogged by the toner.
U.S. Pat. No. 4,491,855 issued on Jan. 1, 1985 in the name of Fujii et al discloses a method and apparatus utilizing a controller having a plurality of openings or slit-like openings to control the passage of charged particles and to record a visible image by the charged particles directly on an image receiving member. Specifically disclosed therein is an improved device for supplying the charged particles to a control electrode that has allegedly made high-speed and stable recording possible. The improvement in Fujii et al lies in that the charged particles are supported on a supporting member and an alternating electric field is applied between the supporting member and the control electrode. Fujii et al purports to obviate the problems noted above with respect to Pressman et al. Thus, Fujii et al alleges that their device makes it possible to sufficiently supply the charged particles to the control electrode without scattering them.
U.S. Pat. No. 4,568,955 granted on Feb. 4, 1986 to Hosoya et al discloses a recording apparatus wherein a visible image based on image information is formed on an ordinary sheet by a developer. The recording apparatus comprises a developing roller spaced at a predetermined distance from and facing the ordinary sheet and carrying the developer thereon. It further comprises a recording electrode and a signal source connected thereto for propelling the developer on the developing roller to the ordinary sheet by generating an electric field between the ordinary sheet and the developing roller according to the image information. A plurality of mutually insulated electrodes are provided on the developing roller and extend therefrom in one direction. An A.C. and a D.C. source are connected to the electrodes, for generating an alternating electric field between adjacent ones of the electrodes to cause oscillations of the developer found between the adjacent electrodes along electric lines of force therebetween to thereby liberate the developer from the developing roller. In a modified form of the Hosoya et al device, a toner reservoir is disposed beneath a recording electrode which has a top provided with an opening facing the recording electrode and an inclined bottom for holding a quantity of toner. In the toner reservoir are disposed a toner carrying plate as the developer carrying member, secured in a position such that it faces the end of the recording electrode at a predetermined distance therefrom and a toner agitator for agitating the toner.
U.S. Pat. No. 4,743,926 granted on May 10, 1988 to Schmidlin et al and assigned to the same assignee as the instant invention discloses an electrostatic printing apparatus including structure for delivering developer or toner particles to a printhead forming an integral part of the printing device. The developer or toner delivery system is adapted to deliver toner containing a minimum quantity of wrong sign and size toner. To this end, the developer delivery system includes a pair of charged toner conveyors which are supported in face-to-face relation. A bias voltage is applied across the two conveyors to cause toner of one charge polarity to be attracted to one of the conveyors while toner of the opposite is attracted to the other conveyor. One of charged toner conveyors delivers toner of the desired polarity to an apertured printhead where the toner is attracted to various apertures thereof from the conveyor.
In another embodiment of the '926 patent, a single charged toner conveyor is supplied by a pair of three-phase generators which are biased by a dc source which causes toner of one polarity to travel in one direction on the electrode array while toner of the opposite polarity travels generally in the opposite direction.
In still another embodiment disclosed in the '926 patent, a toner charging device is provided which charges uncharged toner particles to a level sufficient for movement by one or the other of the aforementioned charged toner conveyors.
The toner in a device such as disclosed in the '926 patent is extracted from the "tops" of the clouds via the fringe fields that extend into the clouds from around the apertures. The efficiency of toner usage in a charged toner conveyor of the type disclosed in the '937 application is currently limited by the relatively dilute toner density in the "tips" of the toner clouds that are transported thereby.
U.S. Pat. No. 4,814,796 granted on Mar. 21, 1989 to Fred W. Schmidlin and assigned to the same assignee as the instant invention discloses a direct electrostatic printing apparatus including structure for delivering developer or toner particles to a printhead forming an integral part of the printing device. The printing device includes, in addition to the printhead, a conductive shoe which is suitably biased during a printing cycle to assist in the electrostatic attraction of developer through apertures in the printhead onto the copying medium disposed intermediate the printhead and the conductive shoe. The structure for delivering developer or toner is adapted to deliver toner containing a minimum quantity of wrong sign and size toner. To this end, the developer delivery system includes a conventional magnetic brush which delivers toner to a donor roll structure which, in turn, delivers toner to the vicinity of apertures in the printhead structure.
U.S. Pat. No. 4,755,837 granted on July 5, 1988 to Fred W. Schmidlin et al and assigned to the same assignee as the instant invention discloses a direct electrostatic printing apparatus including structure for removing wrong sign developer particles from a printhead forming an an integral part of the printing device. The printing device includes, in addition to the printhead, a conductive shoe which is suitably biased during a printing cycle to assist in the electrostatic attraction of developer passing through apertures in the printhead onto the copying medium disposed intermediate the printhead and the conductive shoe. During a cleaning cycle, the printing bias is removed from the shoe and an electrical bias suitable for creating an oscillating electrostatic field which effects removal of toner from the printhead is applied to the shoe. The toner particles so removed are attracted to the copy medium in areas away from the image areas.
U.S. patent application Ser. No. 375,163 which is assigned to the same assignee as the instant invention discloses a toner recovery system for a direct electrostatic printing apparatus. The toner recovery apparatus disclosed in this application comprises a shutter which is interposed between a printhead structure and an imaging substrate during the removal of wrong sign toner accumulated on the printhead structure. During wrong sign toner removal, toner from a toner supply is made to bombard the side of the printhead structure to which wrong sign toner has accumulated thereby dislodging the toner from the printhead. The shutter prevents the toner from being deposited on the imaging substrate. The dislodged toner is carried away utilizing a vacuum and is transported to a collection container where it is stored for future use.
Prior methods of removing the wrong sign toner as disclosed in the '837 patent necessitated pausing between prints resulted in deposition of the toner on the imaging substrate causing undesirable toner smudges. Not only did the cleaning cycle slow the imaging process but it resulted in wasted imaging substrate because the imaging substrate containing the smudges had to be discarded. Also, the toner deposited on the imaging substrate outside of the image area was wasted.